1. Field of the Invention
The present invention relates to a device for and a method of driving an electric motor and, more particularly, to the device for and the method of driving the electric motor based on a detected position of the rotor.
2. Description of the Related Art
For driving a brushless DC motor including a three-phase stator winding comprising three coils each corresponding to a U-, V-, or W-phase, generally, the position of a rotor employed in the brushless DC motor is detected by an induced voltage generated in the coil in which current is not supplied (the phase of this coil is referred to below as the "non-conduction phase"). The brushless DC motor is driven by changing over the coils in which current is supplied to drive the motor (the phases of these coils are referred to below as the "conduction phase") based on the detected rotor position. However, immediately after the start-up, the rotor position cannot be effectively detected since the rotating speed of the brushless DC motor is so low at that time that the induced voltage generated in the stator winding of the non-conduction phase is insufficient. Therefore, a "synchronized operation" has been preformed at the time of start-up, in which the conduction phases are changed over at specified timing regardless of the rotor position of the brushless DC motor to drive the motor. Also, with the rotating speed of the brushless DC motor higher than a specific value, a "position detecting operation" is performed, in which the brushless DC motor is driven by changing over the conduction phases based on the rotor position.
However, the synchronized operation requires an over-excitation operation and, therefore, a problem has been experienced in that a large current flows in the brushless DC motor. Also in the synchronized operation, the current flowing in the brushless DC motor becomes unstable, causing another problem of increased vibrations of the brushless DC motor.